This invention relates to apparatus for compensating reactive power in a three phase network in general and more particularly to a method and apparatus for carrying out such compensation using two converters.
Line commutated thyristor converters are well known in the art. Such converters are described in detail, for example, in the book "Line Commutated Thyristor Converters" by G. Moltgen [Siemens AG and Pitman, 1972] and in the book Thyristor Phase-Controlled Converters and Cycloconverters by B. R. Pelli [Wiley -- Inter-science, 1971]. Line commutated short-circuited converters have been used as reactive load converters. A converter of this nature having controlled rectifier branches which can be fired and extinguished can operate with a firing delay of approximately +90.degree. to present an inductive reactive load and with an advanced firing point of approximately 90.degree. to act as a capacitive reactive load. However, in the transition between presenting an inductive reactive load and a capacitive, reactive load, the entire range of active loads must be traversed continuously. This is given by the range of the firing angles between the two firing angle limits mentioned above. The d-c current when making such a traverse increases so fast that a converter which is short circuited through a choke cannot be considered as a universally usable reactive load converter. A further problem is presented by the fact that where choke filtering of the d-c current is used along with line commutation and controlled rectifier branches which are extinguishable, the current in the choke cannot fall below a certain minimum value if breaks in the d-c current are to be avoided.
In view of these limitations, in a converter controlled over its full range, the need for a universal apparatus useful in compensating reactive power and which permits a fast and continuous transition between an inductive and a capacitive reactive load becomes evident. Furthermore, there is a need for an efficient method of operating such an arrangement.